Polyethylene naphthalate (PEN) is a polyester useful in film, fiber and packaging applications. High molecular weight PEN is produced in a two-step process involving (a) melt polymerization to a relatively low molecular weight, amorphous polymer and (b) solid-state polymerization of this polymer to a high molecular weight polyester. The amorphous polymer is generally formed into the shape of pellets or chips for favorable solid-state reaction rates.
It is desirable to convert the amorphous PEN polymer to the crystalline state prior to solid-state polymerization, because the sticking temperature of the crystalline polymer is higher than that of the amorphous polymer, and use of the crystalline material will thus reduce the tendency of the pellets to stick together as a solid mass during solid-state polymerization.
The crystallization process involves heating the amorphous PEN polymer to its crystallization temperature (about 180.degree.-200.degree. C.). As the temperature is raised, however, the polymer passes through its sticking temperature (about 140.degree. C.). In comparison with other polyesters such as polyethylene terephthalate, PEN pellets crystallize rather slowly, and clumping of the pellets will often occur before the crystallization process is complete.
It is therefore an object of the invention to provide a PEN crystallization process which lowers the crystallization temperature and avoids the problem of sticking or clumping of the PEN pellets.